Power transistors with heavy current carrying capabilities are used in a wide variety of applications including, for example, the switching of current through the windings of an induction motor at a rate established by a switching signal source which in turn may be controlled as a function of desired motor speed. Many other examples of power transistor applications will occur to those skilled in the electronics art.
It is frequently highly desirable to utilize a power transistor in an applcation, such as the application mentioned above, where rapid and precise transitions between full ON and full OFF conditions of operation can be achieved. As is well known, the voltage between the collector and base elements (V.sub.CB) of the transistor determines a transition from an unsaturated level to a hard saturation level. The unsaturated condition provides fast switching times but results in excessive system power dissipation. Hard saturation, while providing low power dissipation during the ON time, causes excess minority carriers in the base region of the transistor resulting in undesirable storage and fall times. In short, it is desirable to be able to control the operation of the power transistor in a switching application to balance the desired switching time of the transistor with the system power dissipation and transistor dissipation. This balance can be achieved by maintaining the transistor in a "quasi" saturation condition; i.e., around the area of saturation (V.sub.CB =0 volts).
A class of circuits known as "Baker clamps" is commonly used to maintain a desired collector base voltage level; see for example, "Baker Clamps, Traditional Concepts Updated for Third Generation Power Transistors" Powerconversion International, July/August 1984, pages 40-45.
The traditional Baker clamp provides its effectiveness at the expense of added drive transistor collector current and higher base drive power dissipation. In adddition, variations in circuit parameters require higher base drive source voltages, typically 7 or more volts, to insure that adequate regulated base current is available to turn ON and sustain the transistor. Any variation in this voltage results in significant degradation of the base drive performance and increased power dissipation. The circuit "solid state drive" referenced in the above-mentioned article uses an NPN and PNP transistor in a common emitter configuration driving the drive transistor. With the base of each tied together the base to emitter junctions of the base drive transistors form the diodes in the base portion of the conventional Baker clamp. This configuration is desirable from a parts standpoint but is susceptible to base emitter temperature changes just as the conventional Baker clamp diodes are also susceptible to temperature changes but to a lesser degree. These changes create undesirable changes in the V.sub.CB over the operating temperature range of each component. Additional diodes used to adjust the V.sub.CB further aggravate the temperature problem. Further complications include the inability of the user to conveniently select the desired V.sub.CB due to the incremental voltage changes that each Baker clamp diode brings to the system. In short it is desirable to control the V.sub.CB independent of the system operating temperature. Another disadvantage of the Baker clamp arises out of the fact that excess base drive current is simply routed by a passive diode into the controlled transistor output circuit. This extra output circuit current creates heat and represents power dissipation. By way of example, five amps of base drive may be available to a transistor requiring only 1 amp for saturation. The excess 4 amps may be dumped off by a Baker clamp diode. The base drive must then fall below one amp before any shut-off progress can be made.